Generally a collimating lens for obtaining a parallel beam of rays from a diverging beam of rays is designed such that it can meet the sine condition. For example, in the case of a thin lens, if the radius of curvature r.sub.1 of the first surface and the radius of curvature r.sub.2 of the second surface satisfy the following formulas, the lens satisfies the sine condition, and becomes a collimating lens without a coma aberration. EQU 1/r.sub.1 ={(n.sup.2 -n-1)/(n.sup.2 -1)}(1/f) EQU 1/r.sub.2 ={n.sup.2 /(n.sup.2 -1)}(1/f)
where f represents the focal distance, and n the refractive index. The usual collimating lenses adopt a radius satisfying the above formules or a similar radius. If the material of the lens is glass, as the usual refractive index of glass is n=1.5.about.1.9, the radius becomes -35&gt;r.sub.2 or r.sub.2 &gt;25.
In the far field pattern of the beam emitting from a semiconductor laser, the horizontal mode gives a smaller beam distribution width than the vertical mode, and therefore, the lateral mode .theta..sub." becomes smaller than the longitudinal mode .theta..sub.1. If, in this way, the beam spot is formed on the records of the optical disc, the spot diameter in the direction of the lateral mode .theta..sub." becomes longer, thereby forming an ellipse. Therefore, as shown in FIG. 5, an anamorphic prism 3 is disposed at the front of the semiconductor laser 1 and the collimating lens 2 in order to modify such that a substantially circular pattern of parallel beam should be formed. As shown in FIG. 5, in a pickup for use in an optical memory having an anamorphic prism 3, the necessary condition is that an intensive beam is concentrated onto the recorded surface of an optical disc to form a small spot as far as possible. Then the condition for a collimating lens accompanied to the above fact is how to efficiently derive the beam of rays from a semiconductor laser, that is, how to improve the coupling efficiency and how to increase the actual number NAob of opening of the objective lens.
If the focal distance f.sub.t of the objective lens facing the optical disc is assumed to be 4.0 mm, and its effective diameter to be 4.4 mm, then the number of the openings will be NAob=0.55. If the number of the openings of the collimating lens is needed by more than 0.3, then the effective diameter of the collimating lens can be made to be 4.4 mm, and the focal distance f.sub.cor to be 7 mm to obtain the number of openings NAcor of 0.3143.
If the semiconductor laser LT024 manufactured by Sharp Corporation is used, the radiation characteristics of the laser becomes as follows: EQU 8.ltoreq..theta..sub." .ltoreq.14 EQU 20.ltoreq..theta..sub.1 .ltoreq.38 (A)
Here, if the beam shaping ratio m for .theta. due to the anamorphic prism is determined to be 2.7 times, the following relation is obtained. EQU 21.6.ltoreq.m.theta..sub." .ltoreq.37.8, EQU that is, .theta..sub." .apprxeq..theta.
In light of the above prerequisites, now the coupling efficiency and the actual number of openings of the objective lens will be considered. First, regarding the coupling efficiency, the higher the coupling efficiency is in an optical memory unit, the greater the concentration of the laser beam becomes, thereby speeding up the recording of information. But depending on the said conditions of the beam shaping ratio, the coupling efficiency can be dropped to about 48% which is below the usually required value. If the coupling efficiency is increased possibly up to 100%, a speedy transmission rate can be obtained.
If the above mentioned ordinary collimating lens is used, dispersing of the beam spot reaching the optical disc occurs due to the diverging trend in the radiation characteristics of semiconductor laser, and therefore, there are cases in which the actual number of openings of the objective lens can not be made to be 0.55. If the usual collimating lens (number of opening NAcor=0.3143) and the usual objective lens (number of openings NAob=0.55) are used, and if a semiconductor laser having a radiation characteristics shown by the formula (A) above and an anamorphic prism having the above mentioned beam shaping ratio (m=2.7 times) are used, then the diameter of the beam spot reaching the recording surface of the disc is calculated as shown by the graphs of FIG. 6. In this drawing, the Tan direction indicates the width of the spot in the direction tangential to the track, while the Rad direction indicates the width of the spot in the radial direction of the disc. In this drawing, reference code (a) represents the width of the spot in the tangential direction when .theta..sub.1 diverges up to 20 to 38 degree, and when .theta..sub." is at the minimum angle of 8 degrees. Reference code (b) represents the width of the spot in the tangential direction when .theta..sub.1 diverges up to 20 to 38 degree and when .theta..sub." is at the maximum angle of 14 degrees. Reference code (c) represents the width of the spot in the radial direction when .theta..sub.1 diverges up to 20 to 38 degrees and when .theta..sub." is at the minimum angle of 8 degrees. Reference code (d) represents the width of the spot in the radial direction when .theta..sub.1 diverges up to 20 to 38 degrees and when .theta..sub." is in the maximum angle of 14 degrees. In these graphs, the spot diameter is dispersed to 1.15 .mu.m to 1.32 .mu.m, while the ratio of the lateral dimension to the longitudinal dimension becomes 0.9 to 1.15. Based on this fact, the actual number of openings of the objective lens can be calculated using the following formula: EQU NAob=0.82.times..lambda./spot diameter
(where .lambda. is the wave length of the beam). NAob=0.49.about.0.55 is obtained by substituting the spot diameter 1.15 to 1.32 .mu.m. That is, it is possible that the actual number of openings of the objective lens can be smaller than 0.55 which is the designed value. Therefore, the diameter of the spot of the laser beam formed on the recording surface of the disc is expanded, resulting in that the concentration is lowered, the transmission rate is decreased, and the recording density is lowered.